Over the past several years there has been, and continues to be, a tremendous amount of activity in the area of efficient encoding of speech and voiceband data. A large number of digital coding algorithms are being investigated for a wide variety of applications. For an evolving digital telephone network, a most important application is the possible replacement of the 64,000 bit-per-second (bps) PCM signal (8 bits per time slot, repeated at an 8 kHz rate) for telephony--both in the public switched and private line networks. The reason, of course, is to achieve bandwidth compression. The dilemma for telephony planners is easily posed but not so easily answered--should such a network evolve toward a coding algorithm more efficient than 64 kb/s PCM and, if so, which algorithm is preferable? A number of different digital coding algorithms and related techniques have been proposed heretofore, namely: Adaptive Differential PCM (ADPCM); Sub-Band Coding (SBC); Time Domain Harmonic Scaling (TDHS); vocoder-driven Adaptive Transform Coding (ATC), etc.
As indicated in the co-pending application of D. W. Petr, Ser. No. 343,355, filed Jan. 27, 1982 now U.S. Pat. No. 4,437,087, issued Mar. 13, 1984 and assigned to the same assignee, for a realistic mix of input speech and voiceband data, the ADPCM approach appears to be the most promising. And, as further disclosed in the Petr application, the ADPCM CODEC (coder-decoder) preferably incorporates adaptive predictors. The adaptive predictors and more particularly, the coefficient update circuit utilized by the same, should have two speeds of operation. A first or normal speed for speech and voiceband data and a second, substantially reduced speed for narrowband signals, such as tones and certain voiceband data signals. By reducing the speed of the coefficient update circuit(s), the ADPCM CODEC's performance is significantly improved. If the speed of the coefficient update circuit is not changed, then in order to ensure good performance for narrowband signals (e.g., tones), it is necessary to fix the speed of operation at the lower of the two speeds. This, however, results in degradation of the voiceband data and speech signals, and so the net effect is a degradation in the overall desired performance of the ADPCM operation.
The adaptive predictor circuit disclosed in the Petr application is particularly advantageous in that it is of a relatively simple and yet a quite reliable design. However, it is not as robust as one might desire in the presence of narrowband signals (tones). Other, prior art, adaptive predictors are known to be more robust, but unfortunately they are typically of complex design.